1. Technical Field
The present document relates to an electromagnetic transponder, that is to say a transmitter-receiver device capable of interchanging data contactlessly and wirelessly. It also relates to a system comprising a contactless reader and such a transponder. It is particularly suitable for a transponder with no standalone power supply, of the contactless integrated-circuit card type. Finally, it relates to a secure method of contactless data interchange based on such a transponder.
2. Description of the Related Art
Certain contactless portable objects, such as certain integrated circuit cards called contactless cards, operate by remote power supply, in other words by contactless power supply. Such an object is activated in the presence of a certain electromagnetic field produced and sent by a reader with which it is supposed to communicate. Hereinafter it will be said that such an object is powered contactlessly by a reader when it is in the electromagnetic field generated by a reader, at its resonance frequency. Such an electromagnetic field transmits to the object the energy for the object to operate while conveying the data interchanged with the reader, during a communication called a radiofrequency communication, or more simply called contactless communication, as opposed to the communications that require a physical electrical contact with a reader via an electrical connector.
An electromagnetic field is therefore used both for powering such a contactless object and for establishing a communication between a reader and the contactless object. Since such a communication is established without physical contact between the two communicating devices, there is a risk that a reader enters into communication with a contactless portable object without the knowledge of its owner, for fraudulent purposes or simply by accident. This risk is very problematic because such contactless objects often contain sensitive data such as data associated with the private life of its owner, and can be used for applications in which great security is desired, such as the medium for personal information in a passport, a health card, or for making electronic payments.
The contactless portable objects may therefore be subjected to malicious attacks for the purpose of gathering personal information from their owner, or for carrying out operations such as electronic signatures or electronic payments without their knowledge. This risk is all the greater if the carrier of the contactless object has no means of detecting the presence of the electromagnetic field of the reader and no means of knowing that his object is in the process of carrying out a contactless communication.
To reduce the risk explained above, document U.S. Pat. No. 6,588,660 describes a contactless integrated circuit card, also called smart card, furnished with an antenna, the structure of which makes it inactive at the usual resonance frequency of contactless smart cards. This card further comprises a housing designed so that a user places his finger in order to create an effect that then modifies the properties of the antenna, more precisely its internal interference parasite capacitor, and consequently its resonance frequency, so that it becomes activatable at the usual and standardized communication frequencies of contactless smart cards. This solution is therefore meant to prevent frauds since an intentional action of the holder of the contactless smart card is necessary to allow the card to be powered and to operate normally. However, such an approach has several drawbacks. First, the effect obtained by the placing of a finger on the housing provided on the card is very variable; notably it depends on the ambient humidity and on individuals. In point of fact, the resonance frequency of the antenna of the card depends precisely on its capacitance. The result of this is that the real resonance frequency of such a card after the placing of a finger on the housing provided remains imprecise and variable and the performance of such a card is greatly compromised because of this uncertainty, which might make it difficult or even impossible to use in certain conditions. Moreover, there are attacks which consist in attempting a communication with a transponder by scanning several frequencies until a response is obtained from the transponder. Such a smart card remains totally vulnerable to such attacks.
Thus, there is a need for a contactless communication solution that does not comprise all or some of the drawbacks mentioned above.
More precisely, there is a first need for a solution allowing a secure and reliable contactless communication.
Moreover, there is a second need for a solution allowing a contactless communication of little bulk, compatible with use within an integrated circuit card, of small dimensions like the format of a bank card as standardized.